Optical fibers are drawn from glass preforms which, during their fabrication, are given a desired refractive index profile. This is done by a process whereby vapors of dopant materials such as SiCl.sub.4, GeCl.sub.4, POCl.sub.3 and BCl.sub.3, and mixtures thereof, are entrained in a carrier gas such as oxygen and drawn as a vapor stream into the interior of a glass "starter" tube. The preform starter tube is rotated while a torch repeatedly traverses its length, and as the vapor stream passes through the tube and encounters the band of heat adjacent the torch, a thermal reaction occurs creating oxides which deposit on and fuse to the interior surface of the tube. After numerous torch passes have formed numerous deposition layers of the same or differing compositions, the tube is collapsed into a solid, rod-shaped preform. This process is termed the Modified Chemical Vapor Deposition or MCVD Process.
Once the preform has been made fiber is drawn from it. This is done by applying heat to a lower portion of the preform causing its lower end to soften from which a thin fiber is drawn. Once the fiber is drawn it mirrors a scaled down version of the preform itself, with that portion of the preform formed by the starter tube becoming the fiber cladding and with that portion formed by vapor deposition layers becoming the fiber core.
As is also well known optical fibers may be manufactured with their core having a uniform index of refraction somewhat higher than that of their cladding, or with their core having a graded index of refraction which, for example, increases parabolically from the area adjacent the cladding to the center of the fiber. Optical fibers of the graded index of refraction type offer superior bandwidth since those rays of light which travel a serpentine path reflected back and forth off the cladding tend to move along the fiber at the same average velocity as those rays which travel along the core center-line. In the fabrication of an optical fiber preform of the stepped index of refraction type the composition of the vapor stream passing through the tube during the vapor deposition process remains constant. Conversely, a graded index of refraction core requires that the composition of the vapor stream be changed many times so that the various layers of glass being fused to the interior of the starter tube possess slight differences in composition to provide slight differences in indexes of refraction. This compositional change can be effected by control of the vapor stream generation. For example, where the vapor stream is generated by bubbling a carrier gas such as oxygen through several bubblers that respectively house dopant materials such as SiCl.sub.4, GeCl.sub.4, POCl.sub.3 and BCl.sub.3 in liquid form, the heat inputted into the bubblers may be changed to increase or decrease the rate at which any particular dopant is vaporized. The various resultant blends then produce glass layers of different indexes of refraction.
A common problem experienced with the just described process is that the preform starter tubes tend to vary in size and shape. This is virtually an inherent characteristic of glass tubes made by extrusion or free-draw. As a result it is necessary to measure each starter tube and to construct and execute vapor stream generation control programs specific to many different starter tube sizes. In other words, a deposition program must be made for each starter tube having a mean cross-sectional wall area over its entire length different from that of a standard size. The development and implementation of chemical vapor deposition control programs for various size starter tubes has proved to be tedious and difficult to execute and monitor, particularly in a manufacturing environment. It thus would be of tremendous benefit if a method could be devised for fabricating optical waveguide fiber preform starter tubes of uniform size.